Photographic half-tone contact screens



Sept. 27, 1966 H. MIDDLEMISS 3,275,445

. PHOTOGRAPHIC HALF-TONE CONTACT SCREENS Filed July 15, 1963 5Sheets-Sheet 1 B4 -IOO- INVENTOQ 7W 2m Wig ATTORNEYJ Sept. 27, 1966 H.MIDDLEMISS 3,275,445

PHOTOGRAPHIC HALF'TONE CONTACT SCREENS Filed July 15, 1963 5Sheets-Sheet 2 I33 LINES PER INCH 93/ LINES PER INCH Q W'MW 5W A: %ZIVLA-r-roRNEYSi p 27, 1966 H. MIDDLEMISS 3,275,445

PHOTOGRAPHIC HALF-TONE CONTACT SCREENS Filed July 15, 1963 5Sheets-Sheet 5 \NVENTOR W {WW ATTORNEYS United States Patent 3,275,445PHOTOGRAPHIC HALF-TONE CONTACT SCREENS Herbert Middlemiss, Nottingham,England, assrgnor of one-half to W. H. Howson Limited Filed July 15,1963, Ser. No. 295,101 Claims priority, application Great Britain, July17, 1962,

27,340/ 62 14 Claims. (Cl. 96116) This invention relates to photographichalftone contact screens of the cross-lined type for use in convertingan image having a variety of continuous tones into differently sizeddots, for the preparation of half tone printing plates.

Heretofore such screens have varied in fineness by having from about 65to up to 300 lines to the inch with corresponding variations insensitivity of adjustment to the original image and resolving powerwhich determines the quality of reproduction of the original image. Itwill be appreciated that there is a limit to the number of basic linesthat the screen can have before it becomes impracticable so that therehas previously been a limit to the quality of reproduction that could beobtained.

The lines on such contact screens are usually made-up from a series ofdots, the dots varying in density from their centers to their edges andthe lines being crisscrossed so that a chequer board pattern isproduced.

One method of making such screens is by projecting light through acamera screen, that is a glass screen provided with a criss-crosspattern of lines, onto a photosensitive film, the light passing throughany square formed by the lines being vignetted to produce the subsequentdot which forms one of the row of dots which produce the lines on thecontact screen, and such contact screens are usually used in directcontact with the photosensitive material on which the final image is tobe produced.

According to the present invention a contact screen has a pattern oflines of spaced dots in which the centre of each dot is in the form of acentral core having a midpoint of maximum density which is surrounded bya pattern of minor dots of smaller area, these having lessening densitywith displacement from the central core, the minor dots being spacedapart in a regular pattern to provide areas of minimum density betweenthem.

Thus in the present invention each of the dots of normal type the edgesof which are of diminishing density is replaced by a dot of the kind setforth above in which the area of diminishing density is pierced in aregular pattern by areas of very low density through which light canpass.

It has been found that in practice the areas of low density allow lineson the final image to grow in the required direction so that favorabledefinition of fine detail is achieved. The general dots which form theimage remain but they tend to be shaped in accordance with the minordots, although little of the pattern of the minor dots remain visible inthe finished reproduction.

Preferably the minor dots cease or are of such density as to almostcease after a predetermined distance to provide an area of minimumdensity between adjacent dots which is of the same area as the centralcores, and the minor dots may be arranged in rows which are parallelwith the rows of dots forming the screen, each minor dot beingsubstantially square and adjacent minor dots having their cornersadjacent.

Preferably each central core is in the shape of a number of minor dotsjoined together along their sides, the areas of minimum density being inthe shape of the spaces left between a number of minor dots.

In a method of making a contact screen as set forth above the pattern ofdots is achieved by coordinating two basic patterns of dots one of whichhas more lines of dots to the inch, and in which the dots are of smallerarea than the other in which the dots correspond to the cores of thefinal pattern. Thus one of the basic patterns of dots corresponds to thehigh density cores and the other corresponds to the rows of minor dotsof lessening density with displacement from the central core.

In a convenient arrangement one basic pattern of dots has 133 lines ofdots per inch and the other has 931 lines per inch.

Preferably the dots on both basic patterns are substant-ially square.

A convenient method of making a contact screen according to theinvention comprises exposing a photosensitive material to a firstpattern of light projected through a cross-lined screen, said pattern oflight corresponding to an area of the pattern of dots and minor dotswhich forms the eventual screen and then projecting a second pattern oflight in place of the first in the form of a low density image of a dotor dots corresponding in area and position to the area of a larger dotor dots in the basic larger pattern projected initially, andsubsequently developing the photosensitive material.

Preferably the first pattern of light corresponds to the pattern on thescreen at the area comprising a single large dot, and provides a centralarea of light intensity which is surrounded bya number of areas of equalin- "ice - tensity but smaller area which are spaced apart from oneanother in both linear directions in a regular pattern. Thus the firstpattern of light projects an image which corresponds to the eventualpattern on the contact screen but without the minor dots of smaller areabeing of lessening density with displacement from the central core.

In a preferred method of projection the first pattern of light isachieved by shining the light from a light source through one of a pairof interchangeable opaque stops which are provided with apertures soshaped as to produce the patterns of light required.

The other stop will be used to control the second pattern of light andthis stop may be provided with apertures in the form of concentric bandswhich are 'of diminishing width from the centre to the edge of the stop.Thus this stop provides a light pattern which simulates a vignetted dotobtained through a normal screen, and the diminishing intensity towardsthe edges of the spot produce the required diminishing intensity of theminor spots around the central core.

The invention may be performed in various ways buta screen according tothe-invention will now be described together with one method ofproducing it, with reference to the accompanying drawings in which,

'FIGURE 1 shows a portion of the pattern on a screen according to theinvention,

FIGURE 2 is a diagrammatic illustration of part of they screen,

, FIGURE 3 is a graph showing the density of each dot on the screen inrelation to the width of the dot,

FIGURE 4 is a diagrammatic illustration of the basic. principle which isused in the production of a screen,

FIGURE 5 is a diagrammatic illustration of the method used for producinga number of dots on the screen and,

FIGURE 6 is another diagrammatic illustration of the method of adjustingthe intensity of the dots.

\FIGURE 1 shows a portion of ascreen according to the invention, fromwhich it will be seen that each dot comprises a central core E which issurrounded by a number of smaller square shaped dots F. The density ofthe central core and of the minor dots 1F lessens with displacement fromthe midpoint of the central core although this is not visible in theblack and white line drawing,

however the minor dot at each corner is of such low density'that it doesnot re-produce 'so that areas of density whichlare of the same shape asthe central cores are produced between adjacent dots. V v

A section of density through each dot will be asshown in FIGURE -3. Inthis figure'which corresponds to a cross section 111 -111 through thedot E on FIGURE ,2 each" minor dot is indicated by the reference letterB. The central core comprises the three dots B B B whicharesubstantially opaque on the screen the dot B being the midpointofmaximum density. The dots B and B are the minordots on each side of thecentral core and as will be seen their density is considerably less thanthat of a central core'itself, and the dots B B have a density which islower. again. As will be seen the density between each of the dots isreduced.

FIGUREZ illustrates the method by which the pattern arranged as 1 3-3lines to the inch, two of these lines being indicated at A and A andthese lines also corresponding to two of the smaller spaced lines whichare formed by small square or diamond shaped dots as indicated at B, B

The lines indicated at A,.A are crossed by similar linesC, C at rightangles thereto, these similar lines also co-inciding with the smallsquare or diamond-shaped dots which are indicated at D, D and the linesC, C A, A intersecting at points such as E, E, E and E Thus thecentresof the dots of the larger spaced lines are at E, E E 1i". and as aresult of the process to :be dfir; scribed central .cores ofmaxiumintensity are formed at these positions as shown. 7

Combined with this basic screen arrangement indicated by the lines C, CA, A there is a secondary screen arrangement the lines of which areprovided by'small dots similar to those indicated at B and which provide931 lines to the inch. Six of these lines are indicated at F to F andare made up of square or diamond shape dots,

such as G, G which, as mentioned above are similar to I the dots B.Similarly secondary lines such as H to H are disposed at right angles tothe lines F to F and are also made up of square or diamond shape dotssuch as In the screen pattern illustrated some of the dots H or F are ofsuch low density as to substantially cease at the points I to J toprovide areas of minimum densty, the reason for this being asset forthabove when discussing the cross-section of the major dots which form thescreen as shown in FIGURE 3. I

it will 'be appreciated that the minor dots B, 'D, G, I, act to providethe shading of density which would normally surround a vignetted dot ona 13'? line per inch' screen. so that although a gradual shading isprovided from one dot to the next as they are displaced from themidpoint of the central core .there is an 'area of lesser densitybetween each minor dot.

. The screen as shown in FIGURES '1, 2 and 3 can be the minor dots endsat a predetermined distance from made by the application of thearrangement shown in FIGURE 4. Inthis arrangement alight. source,indicated atK, can shine throughan opaque, stop L whichis provided witha pattern of apertures which correspond to the surrounding minordots-andcore of a single major dot on the contact screen. In this figure thevarious points B to B of greaterintensity of light. intensity on FIGURE3 are indicated in a corresponding row of apertures across the stop.Part of a crossline screen, such as If a number of apertures areprovided in the photographic screen M, as shown in FIGURE15 then acorre-.

sponding number of patterns similar to N will be-projected, and forsimplicity only the central aperture corre-. Thus'it will be sponding toB is shown in this figure. seen that if sufficient apertures areprovided in the screen M a contact screen of the desired size can 'beproduced in this manner. v

The latent image produced in the photosensitive material by theforegoing method will show little if any of the required 1 grading ofthe minor dots from thecenter to the edges of the major dots. initiallymade of low density and subsequently strengthw ened in the requiredpattern by the following method;

which grades the density of the dots and fills in' between them as shownin FIGURE 3.

In orderto grade .the density of the :dots and in; s between them asshown .in FIGURE 3, the film is-left in position afteran initialexposure to the light source and the stop L in FIGURE-4 isreplaced Ibya-stop O as shown in FIGURE 6. This stop 0 is provided with a series ofapertures in the form of concentric bands P which are of diminishingwidth from the centers to the between the points B3, B and B in FIGURE 38.1'61S1lbfl stantially filled into provide a center core having ,a

midpoint of maximum density and sides. which are of diminishingintensity; 7

In FIGURE 6 thenormal vignetting type dots of V 133 line screen areindicated at Q.

It will be appreciated {that there are other ways of I making a screenaccording to the invention, andithat it would be possible to arrangestop Lito include morethan one basic dot. made of any convenient colourwhich is required.

What I claim is: 1

1. A photographic half-tone contact screen comprising a member having apattern of linearly arranged spaced first dots each of a predeterminedarea, each first dot being in the form of a centralcore having amidpoint of maximum density which is surrounded bya pattern of minordots each of a smaller area than said predetermined area, said minordots. lessening in density with displacement from ,the central core, andthe minor dots. being spaced apart in a regular pattern'to provide areasof minimum density between themselves.

' 2. The photographic half-tone contact screen as defined:

in claim 1 whereinfthe intensity of at least some of the central coresto define areas of minimum density between adjacent minor dots, and saidlast-mentioned areas are substantially identical rareawise to the areasof said first dots.

3. The photographic half-tone contact screen as defined.

in claim 1 wherein each central .core is definedby a number of pluralsided minorqdots, the last-mentioned minor dotsbeing positioned inside-by-side relationship,: and the areas of minimum density being inthe shape of;

' a square left between a number ofsa-id last-mentioned minor dots.

being in corner-tocorner adjacent relationship.

It is, therefore,

Moreover the contact screen may be I 5. The photographic half-tonecontact screen as defined in claim 2 wherein said minor dots arearranged in rows which are parallel to rows of said first dots, eachminor dot being substantially square, and adjacent minor dots being :incorner-to-corner adjacent relationship.

6. The photographic half-tone contact screen as defined in claim 3wherein said minor dots are arranged in rows which are parallel to rowsof said first dots, each minor dot being substantially square, andadjacent minor dots being in corner-to-corner adjacent relationship.

7. A method of making a contact screen comprising the steps of providingan element having a coating of light-sensitive material, forming firstand second light patterns of lines of dots in which there are more lightdots per inch and of a smaller area in the first pattern than in thesecond pattern and the light dots of the second pattern correspond inconfiguration to the final pattern of the contact screen, exposing thelight-sensitive coating by exposing the same to the first and secondlight patterns, and thereafter developing the light-sensitive element.

8. The method of making a contact screen as defined in claim 7 in whichthe first light pattern has 133 lines of dots per inch and the otherpattern has 931 lines of dots per inch.

9. The method of making a contact screen as defined in claim 7 whereinthe dots of both patterns are substantially square resulting in theexposure of like square areas of the light-sensitive element.

.10. The method of making a contact screen as defined in claim 7 inwhich the light-sensitive coating of the element is first exposed to thefirst light pattern and is thereafter exposed to the second lightpattern.

11. The method of making a contact screen as defined in claim 7 in whichthe first pattern of light corresponds in configuration to a :portion ofthe contact screen which forms a single large dot and provides a centralarea of light intensity which is surrounded by a number of areas oflesser intensity but smaller size which are spaced apart from oneanother in two linear directions and in a regular pattern.

12. The method of making a contact screen as defined in claim 7 whereinone of the lines of dots of the first light pattern is in intersectingrelationship to one of the lines of dots of the second light pattern.

13. The method of making a contact screen as defined in claim 7 whereinthe first light pattern is iormed by passing light from a light sourcethrough one of a pair of interchangeable opaque stops which is providedwith apertures shaped to provide the pattern of light required.

14. The method of making a contact screen as defined in claim 7 in whichthe second pattern of light is formed by passing light from a lightsource through one of a pair of interchangeable opaque stops which isprovided with apertures in the form of concentric bands which are ofdiminishing width from the centre of the stop to the edges thereof.

References Cited by the Examiner FOREIGN PATENTS 128,772 8/1948Australia.

NORMAN G. TORCHIN, Primary Examiner.

ALEXANDER D. RICCI, Examiner.

R. H. SMITH, Assistant Examiner.

1. A PHOTOGRAPHIC HALF-TONE CONTACT SCREEN COMPRISING A MEMBER HAVING APATTERN OF LINEARLY ARRANGED SPACED FIRST DOTS EACH OF A PREDTEREMINEDAREA, EACH FIRST DOT BEING IN THE FORM OF A CENTRAL CORE HAVING AMIDPOINT OF MAXIMUM DENSITY WHICH IS SURROUNDED BY A PATTERN OF MINORDOTS EACH OF A SMALLER AREA THAN SAID PREDETERMINED AREA, SAID MINORDOTS LESSENING IN DENSITY WITH DISPLACEMENT FROM THE CENTRAL CORE, ANDTHE MINOR DOTS BEING SPACED APART IN A REGULAR PATTERN TO PROVIDE AREASOF MINIMUM DENSITY BETWEEN THEMSELVES.